If stored in a dark cool place it'll last a long, long time. Now once you put it on your reel you will probably want to change it once a month if you are fishing quite a bit if not more often. If you don't fish much you might stretch it to every six months.
I have some big bulk spools of mono that I keep in some cabinets inside my house and that stuff is 5 years old and is still fine.
I have one reel I bought when I was 16. I'm 34 now and I've never changed the line on it. It's getting pretty bare, but the line that's left is still pretty strong. I wouldn't recommend going this long, but it gives you an idea how long it CAN last. - Coach
I got a deal on this spool of Stren Magnathin I could not pass up. As the picture indicates, it's a big spool. I store it in a closet in the dark inside the house. Ask me on my death bed how it held up - LOL.